In vCenter Server 5.1 with vCenter Single Sign On, the way you set the vCenter Server administrator user depends on your vCenter Single Sign On deployment.

In vSphere versions before
vSphere 5.1, vCenter Server administrators are the users that belong to the
local operating system administrators group.

In vSphere 5.1, when you
install vCenter Server, you must provide the default (initial) vCenter Server
administrator user or group. For small deployments where vCenter Server and
vCenter Single Sign-On are deployed on the same host machine, you can designate
the local operating system group Administrators as vCenter Server
administrative users. This option is the default. This behavior is unchanged
from vCenter Server 5.0.

For larger installations,
where vCenter Single Sign-On and vCenter Server are deployed on different
hosts, you cannot preserve the same behavior as in vCenter Server 5.0. Instead,
assign the vCenter Server administrator role to a user or group from an
identity source that is registered in the vCenter Single Sign-On server: Active
Directory, OpenLDAP, or the system identity source.